The present invention relates to a neck straightener for a stringed instrument such as a guitar to be used in a neck part of a stringed instrument and, more particularly, a neck straightener which is formed by assembling two rod members in a parallel arrangement.
Strings of a stringed instrument such as a guitar are extended between a bridge device provided on a body and pegs provided in a head of a neck part of the instrument, and therefore the neck part is warped relative to the strings in a concave form by a tension force of the strings, thereby causing an upward warp. The neck part of this type of a stringed instrument also is warped relative to the strings in accordance with a type of wood material which forms the neck part and a degree of drying thereof, thereby causing a downward warp. In a case of the upward warp, it is difficult to hold the strings at respective frets of a finger plate and, in a case of the downward warp, the strings come into contact with the frets and the stringed instrument cannot be played normally. For this reason, a conventional stringed instrument is provided with a neck straightener in the neck part to straighten the neck part which otherwise would be warped.
A known neck straightener as described above is formed by a truss rod which is inserted into the neck of the stringed instrument, e.g. as disclosed in Japanese Utility Model Application Disclosure Gazette No. HEI. 3-33488. This type of neck straightener includes a problem that it is necessary to form a curved groove in the neck part and the straightening force is weak. Therefore a neck straightener comprising two rod members, which are assembled in parallel, is used in a stringed instrument with a larger number of strings each having a strong tension, such as an electric guitar, and this type of neck straightener is disclosed, for example, in Japanese Patent Application Disclosure Gazette No. HEI. 1-234898. Such neck straightener using the two rod members is advantageous in that one of the two rod members is curved in a direction opposite to the direction of warp of the neck part by increasing or decreasing the length of the second rod member with reference to the first rod member, and a repulsive force produced by such curving is used as a straightening force for warp of the neck part. Therefore, the rigidity of the entire neck straightener can be increased and the repulsive force can be increased as well. The repulsive force is obtained by curving one of two rod members and therefore a stress concentration point which acts on the neck part is the center of the curve of the curved rod member.
However, the strings which apply tension to the neck part are extended between the bridge device and the pegs. Thus, the center of warp which occurs on the neck part is deviated from the center of the neck part to the body side of the instrument and is presumed as being adjacent the border between a heel portion and a straight portion, in view of a difference of strength between the neck part and the body part and the difference of strength between the heel portion which is the base at the body side of the neck part and the straight portion which is extended from the heel portion.
In the case of a concave warp of the neck part with reference to the strings due to the tension of the strings, there is a problem that the stress concentration point, that is the center of a straightening force, is located at a point deviated from the center of warp of the neck part toward the head side, and therefore the neck part cannot be accurately straightened.
In the case of a convex warp of the neck part with reference to the strings, such warp is dependent on the type of wood material or a degree of drying thereof. Therefore, though it is generally difficult to specify the center of warp, it is generally presumed that the center of such warp exists around the center of the straight portion of the neck part and coincides with the center of straightening by the straightener in most cases.
In the case of the concave warp, however, the center of warp may deviate to the head side of the neck part. In such case, the center position of convex warp greatly differs from that of the concave warp. The center of the convex warp may be estimated by a stringed instrument manufacturer during manufacturing processes and such estimation is done taking the type of wood material and the degree of drying into account. Accordingly, there is still a problem that, if the center of convex warp of the neck part substantially differs from the center or concentration point of a straightening force of the neck straightener, accurate straightening cannot be carried out.